Abstract

Members of the Microviridae comprise two subfamilies. The microviruses (Greek for small), which infect free‐living bacteria, are among the fastest known
replicating viruses. Gokushoviruses (Japanese for very small) occupy a unique niche, infecting obligate intracellular bacteria,
such as Chlamydia and Bdellovibrio, or mollicutes, bacteria without a cell wall. All members of the family contain small (4000–6000 bases), circular, single‐stranded
deoxyribonucleic acid (ssDNA) genomes of positive polarity, which are packaged inside small (∼25 nm diameter) T=1 icosahedral capsids. The other
icosahedral, ssDNA virus families: Parvoviridae, Circoviridae, Nanoviridae and Geminiviridae; share most of these properties, suggesting a large super family spanning several domains of life. The most well known member
of the Microviridae, ϕX174, has been extensively used to study the fundamental mechanisms of DNA replication and capsid assembly. The latter
is uniquely dependent on two scaffolding proteins, and has become a model system for experimental evolution.

Key Concepts:

Whilst overlapping reading frames increase the amount of genetic information encoded in small genomes, they do not appear
to significantly impact the ability of the virus to genetically adapt to selective pressures.

Due to the genome's positive polarity, DNA replication must commence before viral genes can be transcribed.

Microvirus DNA replication occurs in three distinct stages: (1) ssDNA is first converted to a double‐stranded molecule, (2)
amplification of the double‐stranded molecule, (3) single‐stranded genomic DNA synthesis and packaging.

Genomic DNA synthesis and packaging are concurrent processes; thus, a genome is not synthesised unless there exists a capsid
in which to package it.

Virion and procapsids structures. (a) Atomic structure of ϕX174, a microvirus. (b) CryoEM image reconstruction of SpV4, a
gokushovirus. (c) CryoEM image reconstruction of the ϕX174 procapsid. The external scaffolding protein is shaded in light
green, the major spike protein in blue, and the viral coat protein in magenta. The triangle highlights an asymmetric unit,
which contains one coat, one major spike and four external scaffolding proteins. The oval, pentagon and triangle depict the
respective two‐, five‐, and three‐fold axes of symmetry. (d) The atomic structure of the four D proteins found in the asymmetric
unit. Subunits D1, D2, D3 and D4 are depicted in green, yellow, red and turquoise, respectively.

Figure 2.

Genetic maps. (a) Linear depictions of microvirus and gokushovirus genetic maps. With the exception of the HA intercistronic
inset, maps are drawn to scale. For SpV4, the genes corresponding to the ϕMH2K and Chp2 genes are given in parentheses. (b)
Circular depiction of the Microvirus genetic map. (c) Transcription map of ϕX174. Promoters and terminators are depicted with
‘P's’ and ‘T's,’ respectively. The subscript letter demarks the location and name of the promoter or terminator. Grey lines
represent transcripts. The width of the line reflects the gene transcript's relative abundance in an infected cell.

Figure 3.

The ϕX174 lifecycle. The roles and functions of the viral proteins are listed in Table . The host cell rep protein is required for proper DNA packaging.

Feige U and
Stirm S
(1976)
On the structure of the E. coli C cell wall lipopolysaccharide core and on its phiX174 receptor region.
Biochemical and Biophysical Research Communications
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Hafenstein S and
Fane BA
(2002)
phi X174 genome‐capsid interactions influence the biophysical properties of the virion: evidence for a scaffolding‐like function for the genome during the final stages of morphogenesis.
Journal of Virology
76:
5350–5356.